Rule 4: Respect the context
Podcast listeners will know that I put a lot of emphasis on the wider historical context within which philosophy was produced. To some extent it should be obvious how necessary this is: how can we understand, say, Plato and Aristotle's political philosophy without knowing something about the political situation of Athens in their day, or understand Hobbes without knowing about English history? But historical context can be relevant in more surprising ways; my favorite example of this is the parallel between early Islamic debates over the eternity of the universe and the contemporary debate over the eternity or createdness of the Koran. (Actually, though I've drawn this comparison in many places including the podcast, I don't know that anyone agrees with me about it, but I still think it's right.)
There are at least two worries we might have here. First, that history of philosophy is turned into something that is more history than philosophy. Sometimes people speak dismissively of the "history of ideas," in which philosophical theories are nothing but reflections of other historical events. But I strongly feel that history of philosophy is both a kind of history and a kind of philosophy. Understanding the historical context will help us understand philosophical arguments, but going through and evaluating those arguments is still a philosophical enterprise.
Second, that this rule makes it nearly impossible to do the history of philosophy. Are we really supposed to become experts, not only on all these philosophers, but also on the whole context they lived in, taking into account everything from political events to social circumstances, economic factors, etc? My answer would, basically, be yes. There is no point at which you can say, "ok, I've learned enough about the historical context, nothing I learn further will help or be relevant." In principle, it is always worth looking at the context more carefully, no matter how well you understand it, just as it is always worth reading the philosophical texts themselves one more time. The limits are imposed by what we can manage in terms of time and expertise -- like some of the other rules I'm proposing, this rule is intended as an open-ended encouragement to strive for an ideal which is not practically reachable.